It's a hypothetical situation that I don't have, but is all too common. Let's say the unit is maybe 4 years old and the company who installed it is still in business just like many "hack" HVAC companies.
This is a DIY site and electrical stuff is generally more within reach of the average DIYer than refrigerant plumbing. In most areas (at least in the US), it is not necessary to have an electrician license to work on the wiring in your own house - how does Home Depot sell so many electrical parts to the general public?
There is a heat pump out there that runs on a VFD just fine. It was a R22 unit converted to run on R134a, so I figure it must have been really oversized to begin with.